1. Field of the Invention
The present invention relates to methods and apparatus for stabilization of a laser cavity, and more particularly concerns laser frequency stabilization having decreased thermal sensitivity.
2. Description of Related Art
Frequency stabilization of a laser, such as a carbon dioxide waveguide laser, for example, may be accomplished by changing laser cavity length. Use of a piezoelectric crystal transducer (PZT) that carries one of the cavity reflecting end mirrors is one arrangement for changing cavity length. The PZT transducer is driven from the output of a dither stabilizing circuit provided in a feedback loop and having as an input an optical or laser output power signal provided by a power detector. The detector is mounted with laser output optics to sample a portion of the output energy beam and provides an output power feedback signal that is fed to the stabilization circuit. The feedback stabilization circuit is effectively an analog phase comparison circuit that compares the phase of change in output power with the phase of a dither signal generated in the feedback stabilization electronics or associated circuitry to provide a control signal for the piezoelectric transducer. The latter, when energized by the output of the feedback circuit, changes effective cavity length, and therefore laser frequency, by an amount that tends to move the laser operating frequency back toward the peak of the laser gain curve, upon which stabilization is centered. One example of such a phase modulator type of frequency stabilization circuit is shown in the patent to Halmos et al, U.S. Pat. No. 4,660,206. The system of the Halmos et al patent, and other similar frequency stabilization systems, requires close control of laser temperature in order to closely control cavity length within the range of the stabilization system. Such systems can operate only over a very small temperature range, namely a magnitude that causes a cavity length change of not more than the maximum travel of the PZT. If the cavity length change is greater, the system must be manually re-adjusted before it will continue to stabilize.
Laser power signature is a graph of the cyclically repetitive variation of output power plotted against change in effective cavity length. As cavity length changes (is "scanned") through one power signature cycle, the output power varies through a plurality of peaks. Each full signature cycle, which may have several power peaks, for many lasers, occurs over a cavity length variation of a few micrometers, and, as change in cavity length continues, the power signature cycle is repeated. The repetition of the laser signature cycle derives from the fact that the laser cavity is a resonant cavity and will attain resonance at a number of different cavity lengths which are integral multiples of a half wavelength of the resonant frequency.
The commonly used device for changing effective cavity length, namely the piezoelectric crystal transducer, can provide only a limited excursion, not more than a few signature cycles. Therefore, when the piezoelectric transducer is used in frequency stabilization devices of the prior art, the laser may experience temperature changes that cause effective cavity length change of more than a few signature cycles (which may be more than the range of the transducer), thereby causing the system to lose its lock on the desired frequency.
To avoid this problem, a hybrid combination of analog and digital arrangements has been employed, which will search and lock onto a desired peak of the laser signature, and having done so, will then stabilize the frequency at such peak. This system is subject to inaccuracy, is relatively slow and exceedingly expensive. This prior system operates to ramp up the PZT driving voltage and stores the maximum laser power output obtained during such ramping up. The system then ramps the PZT voltage down until a power output of ninety percent of the previous maximum is obtained, and uses the PZT voltage of such ninety percent power output as the peak around which the analog dither stabilization is accomplished. The analog dither stabilization used may be a system such as is disclosed in the Halmos et al patent. Such a hybrid system can discriminate only between peaks that have more than ten percent difference in magnitudes, and, having re-acquired the desired peak, must then switch back to analog operation, thereby requiring a combination of digital circuits and analog circuits that are specially designed for the particular operation. These specially designed circuits, their combination and their packaging are not only more expensive, but require greater space for packaging the components.
Accordingly, it is an object of the present invention to provide for frequency stabilization of the laser that avoids or minimizes above-mentioned problems.